With an amber heat health alert in force and forecasters warning of highs near 35C, the instinct is to switch the air conditioner on first thing and leave it running all day. Yet there is a method that costs nothing and keeps your rooms cool for hours, cutting how long the unit actually has to work. The trick is to use your home's thermal mass, the ability of its walls and floors to store the night's coolness and release it slowly through the day.
How your walls bank the cool night air
The principle is as simple as it is effective. When the outdoor air is at its coolest, overnight and in the early morning, throw the windows open and get a through-draught moving across the whole home. Opening windows on opposite sides of the house works best, so the flow crosses the rooms and carries away the heat the walls soaked up the previous day. Masonry walls, concrete floors and solid surfaces soak up that coolness and hold it like a thermal battery.
Then, before the day heats up, shut everything: windows, shutters or blinds and curtains. The timing of that closing matters, because leaving it until mid-morning traps the already warm air rather than the cool. From that point the cool mass of the building slowly gives its coolness back into the rooms, keeping them comfortable well into the afternoon. The heavier the construction, solid brick or stone, the longer it holds; a lightweight or top-floor flat under the roof warms up sooner and will need the air-con earlier.
Stop the sun before it reaches the glass
The second pillar is shading, and it is best aimed at the south and west-facing windows that catch the sun hardest through the middle of the day and into the afternoon. External shading such as shutters, exterior blinds and awnings works far better than anything indoors, because it stops the sunlight before it ever hits the glass. An internal blind only acts once the heat has already passed through the window and largely arrived: it helps, but much less.
It also pays to cut the heat you generate inside. The oven, a raft of appliances left on standby and old incandescent bulbs all add degrees to a room just when you are trying to keep it cool. Pushing the cooking to the evening and switching off what you are not using takes load off both the house and the air conditioner.
Why this quietly lightens the bill
Be clear about one thing: at 35C this method does not replace the air-con. What it does is shave hours off the time the unit has to run, and that running time is exactly what costs you money. A 1 kW unit, at the electricity unit rate of around 26p per kWh under the Ofgem cap running from 1 July to 30 September 2026, costs about 26p an hour, easily over 6 pounds a day if it runs long, on top of the 57p daily standing charge you pay either way. Every hour your cool walls buy you is money left in your pocket.
One last caveat concerns muggy nights. The method works best where the night genuinely cools down, which is true across much of the country. When the night air stays warm and humid, ventilate less and lean harder on daytime shading instead. Understanding how your own home behaves is the first step to holding the cool for longer while spending less. Our guide to coping with heatwaves and our tips on how to save electricity are a good place to start.